This invention relates to a metal-based cubane structure contained in an octanuclear complex which can be used as an electron transfer agent. More specifically, this invention is directed to a redox-active metal-based structure, protected inside an inert coating. This complex, which can be defined as M8(xcexc4-E)4(xcexc-L)12X4, is stable over several oxidation states. The present invention is also directed to a method of making this product from simple starting materials.
Thermodynamic stability is a desired property of materials to be used in most commercial applications. At the same time, however, chemical versatility is typically required for the manifestation of interesting properties or catalytic activity. A combination of these contrasting characteristics is typically achieved by the coating of large surfaces or particles with inert substances, at the macroscopic and microscopic scale, respectively. Such structures are known to naturally occur at the macromolecular level when, for example, metal active centers are protected inside the organic part of metalloproteins.
The complex encompassed by the present invention has a core, which is the source of redox properties, encapsulated in a protective shell. Such a complex provides an ideal building block for the construction of one-, two-, and three-dimensional materials. These materials can be constructed by connecting or bonding units of the complex made in accordance with the present invention by bridging atoms or groups. The bridges can be either bidentate ligands, which replace terminal atoms, or bifunctional substituents, which connect the units through substitution at the 3-, 4- or 5-position of their respective pyrazoles. The advantage of using the inventive complex for the construction of these materials, instead of a single metal atom, or other mono- or polynuclear products, is that the complex can withstand redox manipulation without significant geometrical changes, which would cause the structure of the material to collapse. The inventor has discovered that the structural integrity of the present complex is a function of the way it is composed. Specifically, the desired redox properties are a function of the metal core, while the connections required for the construction of the above proposed materials take place at the outer inert shell.
Accordingly, construction of the 1-, 2-, and 3-dimensional materials leaves the core unaffected. Similarly, redox changes in the core leave the outer structure unaffected. In addition, when the metal atoms employed are paramagnetic, the complexes encompassed in the invention are also paramagnetic, or can become paramagnetic in one of their oxidation states. The materials which will be prepared from the inventive complex will have all the magnetic and redox properties of this building unit, possibly even amplified.
The inventor has surprisingly and unexpectedly discovered that by encapsulating a redox active core inside an inert protective coating, the resulting material retains structural stability over several oxidation states. This result is achieved by separating the center of redox activity, the core, from the outer surface of the molecule, the coating. In some cases, the molecular symmetry of the complexes M8(xcexc4-E)4(xcexc-L)12X4 allows the existence of optically active forms, which can be prepared as racemic mixtures or enantiomerically enriched or enantiomerically pure forms.
To achieve the beneficial properties of stability and versatility previously described, the present invention is directed to molecules having an active center coated with inert substances. More specifically, the present invention is directed to a complex comprising redox-active metal clusters protected inside a chemically inert shell. The invention is also directed to the generation of four additional forms of the complex through electrochemical reduction of the inventive complex. In addition, the present invention is directed to methods of using these structures as building blocks for the construction of durable supercluster assemblies having electron-transfer properties. The invention is further directed to uses of this complex as a dopant in materials, i.e., polymers to impart magnetic or electrical properties to the doped material. The inventive complex may also be used as a contrast agent in magnetic resonance imaging (MRI) applications, for example.
Pyrazolates are convenient bridging ligands or complexing agents for the synthesis of polynuclear products in which, due to the aromatic character of the ligand or complexing agent, the chemical activity is restricted to the metal centers.
The present invention is directed to a material represented by formula (I)
xe2x80x83M8(xcexc4-E)4(xcexc-L)12X4xe2x80x83xe2x80x83(1)
where M is one or more transition metals, a lanthanide, an actinide and M is in the +2, +3, or +4 oxidation state, or two different oxidation states. Preferably, M is Fe3+, Mn3+ or Co3+;
xcexc represents a bridging group, i.e., a bridging ligand or a bridging chalcogenide;
E is a chalcogenide, preferably O, S or Se;
L is a bridging ligand such as a pyrazole, or a pyrazole substituted at any or all of the 3-, 4-, or 5-positions;
X is a terminal ligand, such as Cl, Br or an alkyl group.
The inventive complex can be in a racemic, an enantiomerically-enriched or an enantiomerically-pure form.
More preferably, formula (I) represents a Fe III-complex designated as
Fe8(xcexc4-O)4(xcexc-pz)12Cl4
wherein pz represents a pyrazolato anion, C3H3N2, and
xcexc represents a bridging group, i.e., a bridging pz, or a bridging atom, such as an oxygen atom. The Fe (III) complex optionally has T-symmetry.
The inventor has unexpectedly discovered that the inventive Fe III complex contains a redox-active Fe4O4 core, protected inside a Fe-pyrazolate coating, which is stable over five oxidation states. The Fe4O4-core of the Fe (III) complex is the first example of an all-ferric/oxygen cubane complex. Consistent with the +3 valence of this Fe III complex are the shorter Fe-O bonds of formula (I), i.e., 2.040(4) xc3x85 average, compared to those of its lower-valence analogues.
As stated, the inventor has also discovered that the complex defined by formula (I) is stable over several, preferably five, oxidation states. Stability is defined as the ability of the complex of formula (I) to retain the structure and stoichiometry of its neutral form when it is reduced by one, two, three or four, one-electron processes. In other words, the inventor has discovered that the species represented by (I), (I)xe2x88x921, (I)xe2x88x922, (I)xe2x88x923 and (I)xe2x88x924 are thermodynamically stable under the electrochemical reduction of the complex of formula (I).
The inventor has also discovered that manipulation of the solubility of the inventive complex is possible through substitution at the outer shell. This can be done through substitution at either the positions of the chlorine atoms, or the 3-, 4-, or 5-position of the pyrazoles. While the preferred form of the complex is hydrophobic, i.e., insoluble in water, but soluble in a large number of organic solvents, it can easily become water soluble by attaching hydrophilic groups to its surface through such substitutions. Water-soluble derivatives of the inventive complex may find medicinal use, either in therapeutic or diagnostic applications, for example, as MRI contrast enhancing agents.
The inventive complex, when manipulated in the above stated manner to make it water soluble, allows the complex to be used in a method of generating an image of a mammal. Such a method comprises administering to a mammal, in an amount effective to provide an image, a contrast agent comprising the complex of formula (I).
Additionally, the inventor has discovered that the complex defined by formula (I) can be assembled in a single reactor from simple starting materials that are commercially available.